1. Field of the Invention
This invention relates to improvements in a photo-sensor or solid-state imaging device which is fabricated on a semiconductor single-crystal substrate.
2. Description of the Prior Art
As an imaging device, there has heretofore been employed an image pickup tube of the type in which a photoconductive target operating in the storage mode is scanned with an electron beam. In this case, the use of the electron beam leads to such difficulties such as the requirement of a high voltage and the need for miniaturization. It is a solid-state imaging device or imaging plate that has been devised in order to overcome these difficulties.
FIG. 1 illustrates the principle of a solid-state imaging device. Picture elements 4 are arranged in a checkered pattern, and signals kept in the picture elements are read out by the XY-address system one by one. The respective picture elements are selected i.e., scanned by a horizontal scan signal generator 1 and a vertical scan signal generator 2. Numeral 3 indicates a swich connected electrically to the picture elements, and numeral 5 an output terminal.
As the concrete construction of a photosensitive region for the picture element, there are an example in which a diffused region is directly formed in a Si substrate, and an example which utilizes a photoconductive thin film, etc.
In the example in which the photosensitive region is constructed by forming the diffused region in the Si substrate, each picture element corresponds to the source region of a MOS switch. Since the MOS FET switches which are arrayed in two dimensions occupy a considerable area, this example is not advisable for the construction of the photosensitive devices.
Interconnections running in the vertical and horizontal directions occupy the surface of the sensor, and reduce the effective photosensitive area. These interconnections cause a lowering of the photosensitivity and diminish the signal output, and therefore form a cause for degrading the signal-to-noise ratio (SN ratio).
On the other hand, in the example which utilizes the photoconductive thin film, scanning circuits for the XY-addressing made of the MOS FET switches, etc. are formed on an Si substrate, and the photoconductive thin film is deposited over the scanning circuits so as to provide the light receiving portions. Such examples of the solid-state imaging devices are disclosed in Japanese Laid-open Patent Application No. 95720/1976, etc. FIG. 2 shows a sectional view for explaining the principle of such devices. In an Si substrate 6, diffused regions 7 and 8 are provided as the source and drain of a MOS swi5tch. Numeral 10 designates a gate electrode of the MOS switch, numeral 15 a drain electrode for leading out a signal, and numeral 16 a source electrode. A photoconductive thin film 17 and a transparent electrode 18 are formed over the switching circuit thus constructed. An insulator layer is designated by reference numeral 13.
A capacitance C is formed between an electrode 16 (area S) and the transparent conductive film 18 with the intervention of the photoconductive thin film 17 which is made of a substance exhibiting photoconductivity, for example, Sb.sub.2 S.sub.3, CdS, As.sub.2 Se.sub.3 or polycrystalline Si. Since the electrode pattern is set in the form of a matrix, equivalent capacitors are arranged in the form of a matrix. Since the capacitor includes the photoconductive film therein, it functions as a photosensitive element and forms a picture element. The photosensitive element has its equivalent circuit expressed by a parallel connection consisting of the capacitance C and a variable resistance R the electric resistance of which varies in response to the intensity of light.
The magnitude of the capacitance C is determined by the electrode area S, the thickness t and dielectric constant .epsilon. of the photoconductive thin film 17, and is expressed as C=.epsilon..multidot.S/t. The magnitude of the resistance is inversely proportional to the intensity of light incident upon the electrode face at the particular position. In case where no light strikes, the resistance is regarded as R.apprxeq..infin. though it is also dependent upon the nature of the photoconductive thin film.
A target voltage (V.sub.T) is applied to the transparent electrode 18, and the capacitor upon which no light impinges during one field period holds the voltage V.sub.T unchanged. In a part of the conductive film upon which light impinges, the resistance R decreases in proportion to the intensity of the light, so that charges stored in the capacitance C are discharged and so that the voltage held in the capacitor decreases in proportion to the quantity of light. Letting U.sub.T denote the voltage left after the discharge in one field period, a charging current corresponding to a voltage V.sub.T -U.sub.T flows. Upon completion of the charging, the capacitor is recharged to the target voltage again. The charging current at this time becomes a video signal which corresponds to this field.
In such solid-state imaging device, imaging characteristics such as spectral response, resolution, SN ratio, and lag characteristics are naturally important. Also important are the stability against temperature change, etc. of the photoconductive thin film. More specifically, the transparent electrode needs to be deposited after forming the photoconductive thin film on the Si body. In this case, the substrate needs to be heated to 400.degree.-500.degree. C. when SnO.sub.2 (Sn Nesa) is employed for the transparent electrode, and it needs to be heated to approximately 250.degree. C. even when In Nesa is employed therefor. This is the reason why the stability against temperature change of the photoconductive film is required. The transparent electrode may well be replaced with a semitransparent metal thin film, with which the heating of the substrate is unnecessary. On account of reflection and absorption of light by the metal thin film, however, the photo response which is important in the imaging characteristics is lowered noticeably. This is especially problematic in the imaging device of the structure shown in FIG. 2. In the imaging target of a conventional image pickup tube, a Nesa electrode is formed on a glass faceplate, whereupon a photoconductive film is deposited. Therefore, whether or not the photoconductive film is resistant against temperature change is not a problem at least in the manufacturing process.
The mechanical strength is also important. After depositing the photoconductive thin film, the operations of providing the Nesa electrode and further providing filters etc. in case of a color imaging plate are necessary, so that the mechanical strength is required from the viewpoint of easy handling.
It is required of the photoconductive thin film that the resistivity of the photoconductive thin film is at least 10.sup.10 .OMEGA..multidot.cm. This is because a charge pattern must not disappear due to diffusion within a time interval in which a specified picture element is scanned, i.e., a storage time.
When polycrystalline Si is employed for the photoconductive thin film, particularly the resistivity is low, and the film needs to be split into a mosaic pattern. This renders the process complicated, and simultaneously lowers the available percentage.
A photoconductive thin film made of Sb.sub.2 S.sub.3, As.sub.2 Se.sub.3 or the like has problems in its mechanical strength and its thermal stability, and has been practically unsuitable for use in the imaging device of the structure shown in FIG. 2.
This invention solves the heretofore described difficulties.
The applicant has filed a patent application (Ser. No. 39,580, filed on May 16, 1979) with the U.S. Patent and Trademark Office in relation to the photosensitive face of an image pickup tube etc. employing a photoconductive film made of an amorphous material whose principal constituent is silicon and which contains hydrogen. Also pending in the U.S. Patent and Trademark Office is a patent application (Ser. No. 48,740 filed on June 15, 1979) concerning a photoconductive material whose principal constituent is silicon and which contains hydrogen and carbon and/or germanium.